In the era of big data, high speed wireless communication becomes essential. In many wireless communication devices, power consumption of a voltage controlled oscillator (VCO) becomes one of the more important performance issues.
The VCO includes an LC tank circuit, an active circuit, and a current source. The LC tank circuit includes an inductor circuit and a capacitor circuit coupled in parallel with the inductor circuit.
The active circuit includes a pair of cross-coupled transistors (e.g., NMOS transistors) and provides a “negative resistance” which is needed to compensate for the losses of the LC tank circuit to sustain oscillation of the VCO. The current source provides driving current for the cross-coupled transistors. The LC tank circuit generates a parasitic resistance Rp that represents the resistive loss of the tank inductors L and/or capacitors C. For oscillation to occur, the negative resistance Rneg (e.g., −1/Gm, here Gm is a transconductance of the pair of cross-coupled transistors) provided by the cross-coupled transistors should be greater than (in absolute value) than the parasitic resistance Rp.
The inductor and capacitor circuits in the LC tank circuit are reconfigured for frequency tuning and frequency tuning extension of the VCO, resulting in change of the parasitic resistance Rp therein, so that the negative resistance Rneg to offset (or compensate) the resistive loss due to the parasitic resistance Rp may be varied. If the negative resistance Rneg is not high enough (e.g., Rneg<Rp), the oscillation does not occur or signal quality (e.g., a signal-to-noise ratio, a signal bandwidth, or a phase noise level) is degraded.
On the other hand, if the negative resistance Rneg is too higher than a magnitude needed to compensate the resistive loss, power waste may occur in the VCO. Thus, the negative resistance Rneg needs to be optimized for a specific parasitic resistance Rp in the LC tank circuit, so that optimized driving current is provided to the cross-coupled transistors and thus, power consumption thereof is minimized.